Magneto-inductive flow measuring devices are widely applied in process and automation technology for fluids having an electrical conductivity of at least, for instance, 5 μS/cm. Corresponding flow measuring devices are sold by the applicant in the most varied of forms of embodiment for different areas of application, for example, under the mark, PROMAG.
The measuring principle rests on Faraday's law of magnetic induction and is known from multiple publications. By means of a magnet system secured on a measuring section, a constant magnetic field is produced directed essentially perpendicularly to the flow direction of the conductive fluid. In this way, ions present in the flowing fluid are deflected in opposite directions. The electrical voltage occurring from this charge separation is sensed by means of at least one measuring electrode pair likewise secured in the measuring tube subsection. The sensed voltage is proportional to the flow velocity of the fluid and therewith proportional to its volume flow rate.
The accuracy of a measurement of a magneto-inductive flow measuring device depends, in such case, on many different factors. Some thereof concern the construction per se, such as, for example, the positioning accuracy of the magnet system, or the read out of the measurement signal via the at least one measuring electrode pair as well as the geometry of the electrode pair. Furthermore, the measuring performance and accuracy of measurement have a sensitive dependence on the reigning flow profile of the fluid.
The flow profile, in turn, depends on the Reynolds number, which depends on the flow velocity, the geometry of the measuring tube and its interior surface roughness, on physical and/or chemical material parameters of the fluid, such as, for example, the viscosity, and on the inlet conditions of the fluid flowing in the measuring tube before the measuring section, in the so-called inlet section.
In the case of given flow quantity, or in the case of given volume flow rate, the flow velocity of the fluid is determined by the cross sectional area of the measuring tube. For very low flow velocities, in the case of a sufficiently long, straight, inlet section of the measuring tube adjoining the measuring section, typically a laminar flow profile is present. If the flow velocity, or the Reynolds number, increases, a transitional region is reached, in which the flow is influenced by the smallest disturbances. In this region, a comparatively high measured value deviation is observed. If the flow velocity increases further, then an increasingly turbulent flow profile is present, where the measured value deviation is again comparable with that in the case of a laminar flow profile. Very high flow velocities can, however, disadvantageously lead to the occurrence of cavitation.
A technique for improving the accuracy of measurement with reference to the dependence on the reigning flow profile is a partial reduction of the cross sectional area of the measuring tube in the region of the measuring section. A reduction of the cross sectional area offers the advantage that the flow velocity of the fluid in this region is higher. In this way, the reigning flow profile is conditioned, which leads over a large range of flow velocity to an improving of the measuring performance, or measuring sensitivity. Then, in turn, the inlet section and outflow section of the measuring tube can be shorter, which can be advantageous especially as regards costs associated with material.
Construction of a measuring tube with partially reduced cross sectional area is disclosed, for example, in European patent, EP2600119A1. There, a magneto-inductive flow measuring device is described, wherein cross sectional area of the measuring section is both less than the cross sectional area of the inflow section as well as also less than the cross sectional area of the outflow section following the measuring section. Moreover, selected for the measuring section is especially a rectangular measuring tube profile. Regarding the manufacture of such a measuring tube, it is mentioned that the cross-sectional reduction is achieved by forces acting externally on the measuring tube. However, how in detail the cross-sectional reduction is accomplished and how the forces can be controlled, in such a manner that they will lead to a certain shape of the measuring tube, is not disclosed. Furthermore, it is not explained how to assure that a liner arranged in the interior of the measuring tube is not damaged by the cross-section reduction.
Another method for reducing the cross sectional area of the measuring section involves the application an internal high-pressure forming, the so-called hydroforming method, and is treated in German patent, DE102008057755A1. However, there is also in the case of this method the problem regarding the liner, which can only be installed following the forming of the measuring tube. This is, however, significantly more complicated than the installation of a liner into a measuring tube still having a uniform cross sectional area.